This invention relates generally to the cooling of a variable frequency drive and, more particularly, to the cooling of a variable frequency drive associated with a refrigerant vapor compressor of a transport refrigeration unit.
Power electronic devices are commonly used for controlling and/or manipulating the characteristics, for example the frequency and/or the voltage, of the electric power being supplied to a variety of electrically powered devices. For example, variable frequency drives are commonly used in connection with variable speed motors for controlling the speed of the motor. Variable speed motors are used in connection with compressors, water pumps, fans and other devices. For example, refrigerant vapor compressors, such as, but not limited to scroll compressors, reciprocating compressors and screw compressors, to enable driving the compression mechanism of the compressor at various operating speeds. As the operating speed of the compression mechanism is decreased, the output capacity of the compressor is decreased, and conversely as the operating speed of the compression mechanism is increased, the output capacity of the compressor is increased. The variable frequency drive is operative to vary the frequency of the electric power supplied to drive motor of the compressor, thereby varying the operating speed of the motor, and consequently the operating speed and output capacity of the compressor.
Transport refrigeration units are commonly used in connection with transport containers for maintaining the cargo box wherein a perishable cargo, such as for example, but not limited to, fresh produce, is stowed at a temperature within a specified temperature range to maintain freshness and minimize spoilage during transit. The transport refrigeration unit includes a refrigerant vapor compressor and condenser/gas cooler disposed externally of the cargo box and an evaporator disposed within the enclosed space of the cargo box. The compressor, condenser/gas cooler and evaporator are connected in a refrigerant circuit in series refrigerant flow relationship in a refrigeration cycle. When the refrigeration unit is operating, air is drawn from within the cargo box, passed through an evaporator in heat exchange relationship with the refrigerant circulating through the refrigerant circuit thereby cooling the air, and the air is supplied back to the cargo box.
To achieve precise temperature control while maintaining system operational efficiency, it is necessary to vary the refrigeration output capacity of the refrigeration unit in response to the refrigeration load demand. For example, during temperature pulldown after exposure of the cargo box to ambient temperature such as during loading of cargo into the cargo box, the compressor of the refrigeration unit is typically operated at maximum output capacity. However, during long periods of operation in a temperature maintenance mode following pulldown, the compressor of the refrigeration unit is operated at low capacity, and often at near zero capacity.
One method of varying the refrigeration capacity of the refrigeration unit is to vary the speed of the compressor using a variable frequency drive as discussed previously to modify the frequency of the electric power being supplied to the electric motor driving the compressor. However, employing a variable frequency drive in controlling compressor speed in connection with a transport refrigeration unit presents a challenge in adequately cooling the power electronics of the variable frequency drive to maintain the reliability and the functionally of the variable frequency drive. This challenge is even more complex for transport refrigeration applications, where operational environments, cargo cooling demands and power electronics heat output vary over a wide spectrum.